This invention relates to the field of digital rock physics and, more particularly, to methods to select drill cuttings or other rock fragments for further analysis and to characterize facies occurrence frequency of a depth interval using drill cuttings or other rock fragments.
Estimating rock properties, such as porosity, total organic content, permeability, and composition, and so forth, has substantial significance, such as for characterizing the economic value of reservoir rock formations. Laboratory analysis of rock samples can be difficult and time consuming. Physical lab experiments are difficult to perform due to the size and shape of cuttings. Devices for generating digital images of rock samples have become available. These devices include, for example, computer tomographic (CT) devices, scanning electron microscopy (SEM) devices, and FIB-SEM (focused ion beam combined with SEM) devices.
Along with technological advances to help analyze geologic features, advances in workflow have been created. For example, a workflow has been shown which has three basic steps of (a) 3D CT imaging and/or FIB-SEM (focused ion beam combined with SEM) imaging; (b) segmentation of the digital volume to quantitatively identify the components, including the mineral phases, organic-filled pores, and free-gas inclusions; and (c) computations of TOC (Total Organic Content), porosity, pore connectivity, and permeability in the three axis. Sisk et al, SPE 134582, “3D Visualization and Classification of Pore Structure and Pore Filling in Gas Shales”, 2010. Using FIB-SEM technology, a sample is analyzed in three dimensions by creating a plurality of two-dimensional images. The segmentation process can be done by, assigning gray scale ranges to features, and volumes can be constructed which show three dimensional distributions of these features. Curtis et al, SPE 137693, “Structural Characterization of Gas Shales on the Micro- and nano-Scales”, 2010. The features that are present within the rock can include, but are not limited to, pores, organic matter, and rock matrix.
Large samples of porous rock are required in order to obtain estimates of rock properties such as permeability, porosity, total organic content, elasticity and other properties that are typical of an entire subterranean rock formation or facies. One common sample used to estimate rock properties is a well core. Well cores are very small compared to an entire formation, so multiple well cores are typically taken and analyzed and rock properties are interpolated in between geographic locations of the cores. When rock properties are estimated using digital rock physics, the problem of sample size versus formation or facies size is even more extreme. Digital rock physics techniques for estimating rock properties have the advantage that they can accurately scan and produce digital images of very fine pore structures and they can identify small volumes of organic materials present in the pore structure of the rock. However, it is very time consuming and expensive to digitally scan very large samples to estimate rock properties. For example, shale rocks can have an average pore size of about 0.005 to 1.0 μm and a well core typically can be about 100,000 μm in diameter and 1,000,000 μm or more in length. The volume of such a core is about 8×1015 μm3 while the volume of a single pore in a shale rock is about 5×10−4 μm3, assuming spherical pores that are 0.1 μm in diameter. Thus the volume of the entire sample (core) is almost 20 orders of magnitude (i.e., 1020 times) greater than the volume of a typical pore. The difference in scale between the sample (core) and the pores contained in the sample can complicate pore analysis thereof. Scanning the entire sample at a resolution high enough to identify all of the pores can result in a complete assessment of the pore structure of the sample. However, scanning the entire sample at a resolution high enough to identify all of the pores is not practical due to the time and expense required to do a complete scan.
In addition, some underground formations such as shale rocks can have many very thin facies, sometimes only a few millimeters or centimeters thick. The accuracy of core depth estimates is on the order of 3 meters. Boreholes can be horizontally separated by hundreds or thousands meters on the surface. Each borehole provides a point of information about the underground formation at a specific surface location. The geologist must interpolate between borehole locations to estimate the location of a facies of interest in between borehole vocations. Underground facies typically do not follow straight lines and as such, significant errors in estimating location of facies can occur. Further, with the advent of horizontal drilling the need to have more detailed information about the precise location of facies and facie properties has become more important. It may not be practical to extract horizontal cores from a well bore and vertical cores may provide only limited data. Core analysis is not practical in real time or near-real time. Cores must be extracted and shipped to a laboratory for analysis and this can require many days or weeks to complete. As a result, core analysis can have reduced value to questions that arise at the time a well is being drilled. Therefore, reliance on cores to estimate properties of subterranean formations can have several shortcomings.
The present investigators have recognized that there is a need for reliable and accurate cuttings preparation, categorization, and sample selection features that can be integrated with methods of high resolution analysis of rock samples.